Let's roll up our sleeves and solve this. We have 405 and we're going to multiply it by 125, essentially adding 405 to itself 125 times.
Step 1: 0 + 405 = 405
Step 2: 405 + 405 = 810
Step 3: 810 + 405 = 1215
Step 4: 1215 + 405 = 1620
Step 5: 1620 + 405 = 2025
Step 6: 2025 + 405 = 2430
Step 7: 2430 + 405 = 2835
Step 8: 2835 + 405 = 3240
Step 9: 3240 + 405 = 3645
Step 10: 3645 + 405 = 4050
Step 11: 4050 + 405 = 4455
Step 12: 4455 + 405 = 4860
Step 13: 4860 + 405 = 5265
Step 14: 5265 + 405 = 5670
Step 15: 5670 + 405 = 6075
Step 16: 6075 + 405 = 6480
Step 17: 6480 + 405 = 6885
Step 18: 6885 + 405 = 7290
Step 19: 7290 + 405 = 7695
Step 20: 7695 + 405 = 8100
Step 21: 8100 + 405 = 8505
Step 22: 8505 + 405 = 8910
Step 23: 8910 + 405 = 9315
Step 24: 9315 + 405 = 9720
Step 25: 9720 + 405 = 10125
Step 26: 10125 + 405 = 10530
Step 27: 10530 + 405 = 10935
Step 28: 10935 + 405 = 11340
Step 29: 11340 + 405 = 11745
Step 30: 11745 + 405 = 12150
Step 31: 12150 + 405 = 12555
Step 32: 12555 + 405 = 12960
Step 33: 12960 + 405 = 13365
Step 34: 13365 + 405 = 13770
Step 35: 13770 + 405 = 14175
Step 36: 14175 + 405 = 14580
Step 37: 14580 + 405 = 14985
Step 38: 14985 + 405 = 15390
Step 39: 15390 + 405 = 15795
Step 40: 15795 + 405 = 16200
Step 41: 16200 + 405 = 16605
Step 42: 16605 + 405 = 17010
Step 43: 17010 + 405 = 17415
Step 44: 17415 + 405 = 17820
Step 45: 17820 + 405 = 18225
Step 46: 18225 + 405 = 18630
Step 47: 18630 + 405 = 19035
Step 48: 19035 + 405 = 19440
Step 49: 19440 + 405 = 19845
Step 50: 19845 + 405 = 20250
Step 51: 20250 + 405 = 20655
Step 52: 20655 + 405 = 21060
Step 53: 21060 + 405 = 21465
Step 54: 21465 + 405 = 21870
Step 55: 21870 + 405 = 22275
Step 56: 22275 + 405 = 22680
Step 57: 22680 + 405 = 23085
Step 58: 23085 + 405 = 23490
Step 59: 23490 + 405 = 23895
Step 60: 23895 + 405 = 24300
Step 61: 24300 + 405 = 24705
Step 62: 24705 + 405 = 25110
Step 63: 25110 + 405 = 25515
Step 64: 25515 + 405 = 25920
Step 65: 25920 + 405 = 26325
Step 66: 26325 + 405 = 26730
Step 67: 26730 + 405 = 27135
Step 68: 27135 + 405 = 27540
Step 69: 27540 + 405 = 27945
Step 70: 27945 + 405 = 28350
Step 71: 28350 + 405 = 28755
Step 72: 28755 + 405 = 29160
Step 73: 29160 + 405 = 29565
Step 74: 29565 + 405 = 29970
Step 75: 29970 + 405 = 30375
Step 76: 30375 + 405 = 30780
Step 77: 30780 + 405 = 31185
Step 78: 31185 + 405 = 31590
Step 79: 31590 + 405 = 31995
Step 80: 31995 + 405 = 32400
Step 81: 32400 + 405 = 32805
Step 82: 32805 + 405 = 33210
Step 83: 33210 + 405 = 33615
Step 84: 33615 + 405 = 34020
Step 85: 34020 + 405 = 34425
Step 86: 34425 + 405 = 34830
Step 87: 34830 + 405 = 35235
Step 88: 35235 + 405 = 35640
Step 89: 35640 + 405 = 36045
Step 90: 36045 + 405 = 36450
Step 91: 36450 + 405 = 36855
Step 92: 36855 + 405 = 37260
Step 93: 37260 + 405 = 37665
Step 94: 37665 + 405 = 38070
Step 95: 38070 + 405 = 38475
Step 96: 38475 + 405 = 38880
Step 97: 38880 + 405 = 39285
Step 98: 39285 + 405 = 39690
Step 99: 39690 + 405 = 40095
Step 100: 40095 + 405 = 40500
Step 101: 40500 + 405 = 40905
Step 102: 40905 + 405 = 41310
Step 103: 41310 + 405 = 41715
Step 104: 41715 + 405 = 42120
Step 105: 42120 + 405 = 42525
Step 106: 42525 + 405 = 42930
Step 107: 42930 + 405 = 43335
Step 108: 43335 + 405 = 43740
Step 109: 43740 + 405 = 44145
Step 110: 44145 + 405 = 44550
Step 111: 44550 + 405 = 44955
Step 112: 44955 + 405 = 45360
Step 113: 45360 + 405 = 45765
Step 114: 45765 + 405 = 46170
Step 115: 46170 + 405 = 46575
Step 116: 46575 + 405 = 46980
Step 117: 46980 + 405 = 47385
Step 118: 47385 + 405 = 47790
Step 119: 47790 + 405 = 48195
Step 120: 48195 + 405 = 48600
Step 121: 48600 + 405 = 49005
Step 122: 49005 + 405 = 49410
Step 123: 49410 + 405 = 49815
Step 124: 49815 + 405 = 50220
Step 125: 50220 + 405 = 50625

So, 405*125 = 50625